The heat evolved from individual cells in a microscopic field modifies the thickness of a condensing film of oil on a thin supporting membrane to allow estimation of heat evaluation from individual cells. Granulocyte ingestion of bacteria correlated with increased metabolic heat. Improvement in instrumentation to permit observation by transmitted light and application to specific cell measurements is projected. Measurement of cell heat evolved on application of transforming agents and metabolic stimulants will be evaluated as an index of specific activity.